This invention relates to magnetic materials and, in particular, to the use of rotational symmetry to tailor the magnetic properties of the materials.
The volume of information which can be stored on a computer hard disk has risen by a factor of 107 in the past 40 years and looks set to continue rising at an exponential rate in coming decades. Today""s conventional magnetic materials will be unable to meet the demanding performance requirements of tomorrow""s magnetic data storage industry. One option currently being considered is a synergy of nanotechnology and quantum mechanics to make nanometre scale magnetic particles called nanomagnets. These, by virtue of their extremely small size, possess very different magnetic properties from their parent bulk material. Each nanomagnet is analogous to a giant atom of an artificial element, allowing new magnetic materials to be built up giant atom by giant atom. The rapidly growing field of nanomagnetism may provide among other things advanced replacements for hard disk media and a new generation of high speed, low power, non-volatile computer memory chips.
The most important property of a naturally occurring magnetic element or alloy is its anisotropy. This refers to the presence of preferred magnetisation directions within the material and is ultimately responsible for determining the way in which a magnetic material behaves and the technological applications for which it is suitable. In a conventional magnetic material anisotropy arises from the shape and symmetry of the electronic Fermi surface and so is intrinsic to the particular element or alloy and cannot easily be tailored. In nanomagnets, however, the anisotropy depends not only on the band structure of the parent material, but also on the shape of the nanomagnet. One of the most attractive features of artificial magnetic materials is that their magnetic properties can be engineered by the choice of the shape of the constituent nanomagnets.
According to one aspect of the present invention, a memory element comprises nanomagnets having a rotational symmetry selected in order to provide high remanence and a suitable coercivity.
According to a second aspect of the present invention, a sensor element comprises nanomagnets having a rotational symmetry selected such that they are superparamagnetic and exhibit substantially zero hysteresis so that a magnetisation of the nanomagnets depends only on the current value of applied field and not on the field history.
According to a third aspect of the present invention, a magnetic logic element comprises nanomagnets having a rotational symmetry selected such that they are superparamagnetic and exhibit substantially zero hysteresis so that a magnetisation of the nanomagnets depends only on the current value of applied field and not on the field history.
The devices of the invention are artificial magnetic materials formed on the surface of a substrate, such as a piece of silicon, using a technique such as electron beam lithography. The devices may be in the size range 40-500 nm and in the thickness range 3-10 nm and may be, triangular, or pentagonal geometries which respectively correspond to rotational symmetries of order 3 and 5. The order may, however, be greater. The parent material may be Supermalloy (Ni80Fe14Mo5), which is chosen for two reasons. Firstly, this alloy is intrinsically almost isotropic and so any anisotropy in the nanomagnets must come from their shape. Secondly, Supermalloy and its molybdenum-free relative Permalloy are two of the ubiquitous soft magnetic alloys of industry and research and as such make an effective demonstration of the new and varied properties which can be given to a simple material by nanometric structuring. As described below, we are able to make artificial magnetic materials with an enormously wide range of magnetic properties simply by varying the symmetry of the constituent nanomagnets.